The conversion and I/O functions are impossible to memorise there is so little pattern to them. See the conversion table of methods.

Java is quite inconsistent about caps, e.g.
Beans.isInstanceOf
Character.isWhitespace
Checkbox
Color.RGBtoHSB
Date.getTimezoneOffset
File.mkdirs
javax.mail.part.getFileName()
java.io.FilenameFilter
GridBagConstraints.grid width
java.nio.charset.Charset
hashCode
Hashtable
instanceof
java.lang.Runtime
java.util.TimeZone
JpopupMenu
Keymap
org.omg.SendingContext.RunTime
StreamTokenizer.whitespaceChars
System.arraycopy
java.util.zip.GZIPInputStream
java.util.zip. ZipOutputStream

I don’t mean to imply those choices are all wrong, merely that the spelling choice is not obvious to everyone and that Java is often inconsistent.

Some collections use putAll others addAll, some put some add. You can remember this way, you put with a key or index, but you add to the end.

Iterators and Enumerations are almost identical, but Iterators use shorter names for the methods. In general older classes offer an Enumeration and newer ones offer an Iterator or a Collection. To the newcomer, it seems almost random if a class offers an Iterator or a Enumeration with iterator/ elements, hasNext/hasMoreElements, next/getNextElement.

Finding the length of something is done in a variety of ways: array.length for arrays, String.length(), StringBuilder.length(), StringBuffer. length(), ArrayList.size().

Emptying is done a variety of ways: ArrayList.clear() releases the internal storage rather than clearing it. To recycle the same storage you need to do ArrayList.remove( 0, size-1); StringBuffer. setLength(0) and StringBuilder. setLength(0) recycle existing internal storage for reuse.